At last the day came when the big government test was to be made.Tom had completed his Air Scout and had refined it to a point where even his critical judgment was satisfied.All that remained now was to give Mr.Terrill a chance to see how silently the big craft could fly, and to this end a flight was arranged.

Tom had put the silencer on a larger machine than the one he and Jackson had used.It held three easily, and, on a pinch, four could be carried.Tom's plan was to take Mr.Damon and Mr.Terrill, fly with them for some time in the air, and demonstrate how quiet his new craft was.Then, by contrast, a machine without the muffler and the new motor with its improved propellers would be flown, making as much noise as the usual craft did.

"I only wish," said Tom, as the time arrived for the official government test, "that Mary could be here to see it.She was the one who really started me on this idea, so to speak, as it was because I couldn't talk to her that I decided to get up a silent motor."But Mary Nestor was too grief-stricken over her missing father to come to the test, which was to take place late one afternoon, starting from the aerodrome of the Swift plant.

"First," said Tom, to Mr.Terrill, "I'll show you how the machine works on the ground.I'll run the motor while the plane is held down by means of ropes and blocks.Then we'll go up in it.""That suits me," said the agent."If it does all you say it will do, and as much as I believe it will do, Uncle Sam will be your debtor, Mr.Swift.""Well, we'll see," said Tom with a smile.

Preparations were made with the greatest care, and Tom went over every detail of the machine twice to make certain that, in spite of the precautions, no spy had done any hidden damage, that might be manifested at an inopportune moment.But everything seemed all right, and, finally, the motor was started, while Mr.Terrill, and some of his colleagues from the Army Aviation department looked on.

"Contact!" cried Tom, as Jackson indicated that the compression had been made.

The mechanic nodded, gave the big propeller blades a quarter turn andjumped back.In an instant the motor was operating, and the craft would have leaped forward and cleaved the air but for the holding ropes and blocks.Tom speeded the machinery up to almost the last notch, but those in the aerodrome hardly heard a sound.It was as though some great, silent dynamo were working.

"Fine!" "Wonderful!"

"Wouldn't have believed it possible!"

These were some of the comments of the government inspectors."And now for the final test--that in the air," said Mr.Terrill.

Previous to this he and his colleagues had made a minute examination of the machinery, and had been shown the interior construction of the silencer by means of one built so that a sectional view could be had.Tom's principles were pronounced fundamental and simple.

"So simple, in fact, that it is a wonder no one thought of it before," said a navy aviation expert."It is the last word in aircraft construction--a silent motor that will not apprise the enemy of its approach! You have done wonders, Mr.Swift!" "I'd rather hear you say that after the air test," replied Tom, with a laugh."Are you ready, Mr.Terrill?""Whenever you are."

"How about you, Mr.Damon?"

"Oh, I'm always ready to go with you, Tom Swift.Bless my trench helmet, but you can't sail any too soon for me!"There was a genial laugh at his impetuosity, and the three took their seats in the big craft.Once more the engine was started.It operated as silently as before, and the first good impressions were confirmed.Even as the machine moved along the ground, just previous to taking flight into the air, there was no noise, save the slight crunch made by the wheels.This, of course, would be obviated when Silent Sam was aloft.